[Paper Review] Searches for Higgs Bosons Beyond the Standard Model

A. Sopczak|ArXiv.org|Aug 3, 2004

Particle physics theoretical and experimental studies3 citations

TL;DR

This paper presents comprehensive searches for Higgs bosons beyond the Standard Model using combined data from the four LEP experiments (ALEPH, DELPHI, L3, OPAL), analyzing various extensions such as the MSSM, CP-violating models, and two-Higgs-doublet frameworks. It sets stringent 95% confidence level limits on Higgs boson masses and couplings, with key results including exclusion of large regions of parameter space and strong constraints on invisible decays and exotic Higgs states.

ABSTRACT

Latest results from the combined data of the four LEP experiments ALEPH, DELPHI, L3 and OPAL on Higgs boson searches in extensions of the Standard Model (SM) are presented.

Motivation & Objective

• To extend the Standard Model Higgs boson search program to new physics scenarios beyond the SM using LEP data.
• To set robust exclusion limits on Higgs boson masses and couplings in extended models such as the MSSM and two-Higgs-doublet models.
• To investigate the impact of new measurements—particularly the increased top quark mass—on Higgs constraints in supersymmetric models.
• To explore exotic Higgs decay modes, including invisible decays and fermiophobic couplings, to probe new physics.
• To evaluate the sensitivity of LEP to doubly-charged Higgs bosons and other exotic states via forward-backward asymmetry and direct production.

Proposed method

• Combines data from four LEP experiments (ALEPH, DELPHI, L3, OPAL) over 11 years, with a total integrated luminosity of 2461 pb⁻¹.
• Applies model-independent and benchmark-specific search strategies for Higgs bosons in various extensions, including MSSM, CP-violating models, and two-Higgs-doublet frameworks.
• Uses advanced analysis techniques such as artificial neural networks (ANNs) and b-tagging to enhance sensitivity to rare decay modes.
• Performs dedicated searches for specific final states, including hZ, HZ, hA, and b̄bH, to probe neutral and charged Higgs bosons.
• Applies mass limits and coupling constraints using observed and expected background distributions, with 95% CL intervals.
• Conducts a general MSSM parameter scan and two-Higgs-doublet model scans to derive model-independent limits across broad parameter spaces.

Experimental results

Research questions

• RQ1 What are the 95% confidence level limits on the mass of the Standard Model Higgs boson and its couplings in the presence of reduced production rates?
• RQ2 How do the LEP data constrain the parameter space of the Minimal Supersymmetric Standard Model (MSSM), particularly for light Higgs bosons and the A boson?
• RQ3 To what extent do CP-violating Higgs sectors reduce the sensitivity to Higgs signals, and what are the resulting mass limits?
• RQ4 What are the constraints on invisible Higgs decays and their interpretation in Majoron and MSSM models?
• RQ5 What are the limits on exotic Higgs states such as singly- and doubly-charged Higgs bosons, and on fermiophobic decays to WW, ZZ, and γγ?

Key findings

• The Standard Model Higgs boson is excluded below 114.4 GeV at 95% CL, based on combined LEP data.
• For reduced Higgs couplings, limits on the Higgs mass range from 80 GeV (for b̄b decay with ξ² > 0.04) to 113 GeV (for τ⁺τ⁻ decay with ξ² > 0.2).
• In the MSSM, the general parameter scan excludes mₕ > 87 GeV and mₐ > 90 GeV, with significant constraints in the maximum h-mass benchmark scenario.
• The increased top quark mass significantly reduces previous tanβ limits, especially in the MSSM.
• CP-violating models lead to strongly reduced mass limits, with full CP-mixing reducing sensitivity compared to no mixing.
• Limits on doubly-charged Higgs bosons are mₕ⁺⁺ > 99 GeV (combined DELPHI/OPAL), and for e⁺e⁻ → e⁺e⁻, mₕ⁺⁺ > 700 GeV if hₑₑ > 0.5.